Inviting step family?

It’s FH’s call - if he doesn’t feel the need for them to be there that’s fine. Just invite dad and step mom.

If it's concerning you this much I'd have one more discussion with him about it and then drop it. Explain how you feel it'd be rude to only send invites to his dad and stepmom, you understand where he is coming from,and you wouldn't want any regretful feelings on invites to arise after the wedding. If he says he doesn't want to invite them, accept his wishes and move on. If he has a change of heart, send the invitations.

If they live in the same household then it may be rude - but not always! - but if these are adults living their own lives, it is absolutely fine to not even consider them.